If you’re struggling with mold corrosion from chemicals, moisture, or harsh plastics (like PVC), JIS S136 mold steel is the solution you need. This premium stainless mold steel combines exceptional corrosion resistance with outstanding mirror polishability—making it ideal for medical, food, and high-end consumer product molds. In this guide, we’ll break down its key properties, real-world applications, manufacturing steps, and how it compares to other materials—so you can build molds that last and deliver flawless parts.
1. Material Properties of JIS S136 Mold Steel
JIS S136’s unique performance comes from its high-chromium composition and carefully tuned properties. Let’s explore these in detail:
1.1 Chemical Composition
The elements in JIS S136 work together to boost corrosion resistance, polishability, and durability. Below is its standard composition (per JIS G4404):
| Element | Content Range (%) | Key Role |
|---|---|---|
| Carbon (C) | ≤ 0.08 | Low content reduces carbide formation, enhancing corrosion resistance and polishability. |
| Manganese (Mn) | ≤ 1.00 | Improves hardenability without compromising corrosion resistance. |
| Silicon (Si) | ≤ 1.00 | Enhances strength and resistance to oxidation in high-temperature molds. |
| Chromium (Cr) | 12.00 – 14.00 | The primary element for corrosion resistance; forms a protective chromium oxide layer on the surface. |
| Molybdenum (Mo) | 0.40 – 0.60 | Boosts corrosion resistance in acidic environments; improves high-temperature strength. |
| Vanadium (V) | 0.10 – 0.30 | Refines grain structure, enhancing mirror polishability and dimensional stability. |
| Sulfur (S) | ≤ 0.030 | Minimized to avoid surface defects (e.g., pits) during polishing. |
| Phosphorus (P) | ≤ 0.030 | Kept low to prevent brittleness and ensure uniform corrosion resistance. |
1.2 Physical Properties
These properties determine how JIS S136 behaves in manufacturing and use—such as heat transfer and resistance to warping. All values are measured at room temperature unless noted:
- Density: 7.85 g/cm³ (consistent with most stainless steels, making it easy to calculate mold weight).
- Melting Point: 1450 – 1510 °C (high enough to withstand forging and heat treatment without deformation).
- Thermal Conductivity: 25 W/(m·K) (lower than carbon steel, but sufficient for even cooling in plastic injection molds).
- Coefficient of Thermal Expansion: 11.5 × 10⁻⁶/°C (from 20 to 600 °C; low expansion ensures dimensional stability in heat cycles).
- Specific Heat Capacity: 460 J/(kg·K) (efficient at absorbing heat, reducing plastic cooling time in molds).
1.3 Mechanical Properties
JIS S136 is often supplied in a pre-hardened state (HRC 30–35) for convenience, but it can also be heat-treated to higher hardness. Below are its typical properties after standard heat treatment (quenching + tempering at 500 °C):
| Property | Typical Value | Test Standard | Why It Matters |
|---|---|---|---|
| Hardness (HRC) | 30 – 35 (pre-hardened); 45 – 50 (fully hardened) | JIS Z2245 | Balanced hardness—pre-hardened for easy machining; fully hardened for high-wear molds. |
| Tensile Strength | ≥ 1000 MPa | JIS Z2241 | Handles the pressure of plastic injection or die casting without deformation. |
| Yield Strength | ≥ 850 MPa | JIS Z2241 | Resists permanent damage, keeping molds dimensionally stable for thousands of cycles. |
| Elongation | ≥ 15% | JIS Z2241 | Good ductility, reducing the risk of cracking during mold clamping or polishing. |
| Impact Toughness (Charpy V-notch) | ≥ 45 J (at 20 °C) | JIS Z2242 | Excellent toughness—prevents mold failure from sudden impacts (e.g., part jams). |
| Fatigue Strength | ~450 MPa (10⁷ cycles) | JIS Z2273 | Resists wear from repeated use (key for high-cycle food packaging molds or medical device molds). |
1.4 Other Properties
- Corrosion Resistance: Excellent. The high chromium content forms a passive oxide layer that resists rust, acids, and chemicals (e.g., PVC additives or medical disinfectants)—far better than standard mold steels.
- Wear Resistance: Good to Very Good. Molybdenum and vanadium form hard carbides; for high-wear applications, fully hardening (HRC 45–50) or nitriding boosts wear resistance further.
- Machinability: Fair. Pre-hardened JIS S136 (HRC 30–35) is machinable with carbide tools, but it’s slower to machine than P20 mold steel due to its stainless properties.
- Hardenability: Very Good. It hardens evenly across thick sections (up to 60 mm), so large molds have consistent performance.
- Mirror Polishability: Outstanding. Low carbon and fine grain structure let it achieve mirror finishes (Ra ≤ 0.01 μm)—critical for consumer product molds (e.g., cosmetic containers) or medical devices.
- Dimensional Stability: Excellent. Low thermal expansion and uniform hardening prevent mold warping during machining or heat cycles.
2. Applications of JIS S136 Mold Steel
JIS S136’s corrosion resistance and polishability make it a top choice for demanding industries. Here are its most common uses, with real examples:
2.1 Medical Device Molds
- Examples: Molds for plastic syringes, surgical instrument handles, or diagnostic test kits.
- Why it works: Corrosion resistance stands up to frequent disinfection (e.g., ethanol or steam), while mirror polishability ensures parts are free of bacteria-trapping defects. A U.S. medical manufacturer used JIS S136 for syringe molds—mold life increased by 300% vs. standard steel.
2.2 Food Packaging Molds
- Examples: Molds for plastic food containers, beverage caps, or disposable cutlery.
- Why it works: Resists corrosion from food acids (e.g., tomato sauce, citrus) and cleaning chemicals. A European packaging company used JIS S136 for yogurt cup molds—mold maintenance dropped by 50%.
2.3 PVC or Corrosive Plastic Molds
- Examples: Molds for PVC pipes, vinyl siding, or chemical storage containers.
- Why it works: PVC releases corrosive gases during molding—JIS S136’s stainless properties prevent mold rusting. A Chinese manufacturer used JIS S136 for PVC pipe molds—mold failure from corrosion stopped entirely.
2.4 High-End Consumer Product Molds
- Examples: Molds for smartphone casings, cosmetic jars, or luxury watch components.
- Why it works: Mirror polishability delivers premium surface finishes, while corrosion resistance keeps molds looking clean. A Japanese electronics brand used JIS S136 for smartphone case molds—part defect rates fell to 0.2%.
3. Manufacturing Techniques for JIS S136 Mold Steel
Turning JIS S136 into high-performance molds requires specialized processing to preserve its corrosion resistance and polishability. Here’s a step-by-step breakdown:
- Melting: Raw materials (iron, chromium, molybdenum, etc.) are melted in an electric arc furnace (EAF) with strict control to keep carbon and impurity levels low—critical for corrosion resistance.
- Casting: Molten steel is poured into ingot molds or continuous casters to form slabs. Slow cooling (50–100 °C/hour) prevents internal defects and ensures uniform grain structure.
- Forging: Slabs are heated to 1100–1200 °C and pressed/hammered into mold blanks (e.g., 500x500x200 mm for injection molds). Forging improves toughness and eliminates voids.
- Heat Treatment:
- Pre-hardened Cycle: Anneal (800–850 °C) → Quench (1000–1050 °C) → Temper (550–600 °C) → Final hardness HRC 30–35.
- Fully Hardened Cycle: Anneal → Quench (1000–1050 °C) → Temper (400–450 °C) → Final hardness HRC 45–50.
- Machining: Use carbide tools for pre-hardened JIS S136 (HRC 30–35). Reduce cutting speeds by 20–30% vs. P20 steel to avoid tool wear. CNC machines are recommended for tight tolerances (±0.001 mm).
- Polishing: To achieve mirror finish:
- Start with 400-grit sandpaper to remove machining marks.
- Progress to 1000-grit, 3000-grit, and 8000-grit sandpaper.
- Finish with diamond paste (1–3 μm) using a soft felt pad—this ensures no scratches compromise the mirror surface.
- Surface Treatment (Optional):
- Nitriding: Heat to 500–550 °C in a nitrogen-rich environment. Adds a hard surface layer (HRC 60–65) to boost wear resistance (ideal for die casting molds).
- Electroplating: Chrome or nickel coating for extra corrosion protection in harsh chemical environments.
4. Case Study: JIS S136 in Medical Syringe Molds
A German medical device manufacturer faced a crisis: their standard steel syringe molds were rusting after 50,000 cycles due to steam sterilization, leading to contaminated syringes and product recalls. They switched to JIS S136, and here’s what happened:
- Process: Molds were forged, pre-hardened to HRC 32, machined to syringe cavity geometry, polished to Ra 0.008 μm, and passivated (to enhance corrosion resistance).
- Results:
- Mold life increased to 250,000 cycles (400% improvement) with no rust.
- No more recalls—syringes met strict medical cleanliness standards.
- Maintenance time dropped by 60% (no need to repair rust damage).
- Why it works: JIS S136’s chromium oxide layer resisted steam and disinfectants, while its mirror polishability eliminated bacteria-trapping surface defects—solving both corrosion and contamination issues.
5. JIS S136 vs. Other Mold Materials
How does JIS S136 compare to common alternatives for corrosion-prone or high-precision applications? Let’s evaluate key properties:
| Material | Corrosion Resistance | Mirror Polishability (Ra μm) | Machinability | Cost (vs. JIS S136) | Best For |
|---|---|---|---|---|---|
| JIS S136 Mold Steel | Excellent | ≤ 0.01 | Fair | 100% | Medical molds, PVC molds, food packaging |
| P20 Pre-hardened Steel | Poor | ≤ 0.05 | Excellent | 50% | General plastic molds (no corrosion risk) |
| JIS NAK80 Mold Steel | Good | ≤ 0.01 | Excellent | 80% | High-polish molds (no harsh chemicals) |
| Aluminum Mold Materials (7075) | Poor | ≤ 0.02 | Excellent | 60% | Prototype molds (low-volume, no corrosion) |
| Carbon Steel (1045) | Very Poor | ≥ 0.10 | Excellent | 30% | Low-cost, low-precision molds |
Key takeaway: JIS S136 is the only material that combines excellent corrosion resistance with mirror polishability. It’s more expensive than P20 or NAK80, but the cost is justified for corrosion-prone applications like medical or PVC molds.
Yigu Technology’s View on JIS S136 Mold Steel
At Yigu Technology, JIS S136 is our go-to for clients in medical, food, and chemical industries. Its corrosion resistance solves the biggest pain point—premature mold failure from rust or chemicals—while its polishability ensures flawless parts. We often recommend pre-hardened JIS S136 for faster production and pair it with passivation or nitriding to enhance performance. For businesses prioritizing durability and compliance (e.g., medical standards), JIS S136 isn’t just a material—it’s a way to avoid costly recalls and build long-lasting molds.
FAQ About JIS S136 Mold Steel
1. Can JIS S136 be used for hot runner systems in plastic injection molds?
Yes! JIS S136’s high-temperature stability (up to 500 °C) and corrosion resistance make it ideal for hot runners—especially for PVC or other corrosive plastics. We recommend fully hardening it to HRC 45–50 or adding a nitrided layer to boost wear resistance.
2. Is JIS S136 more difficult to machine than P20 steel?
Yes, JIS S136 is slower to machine than P20 because of its stainless properties (higher chromium content increases cutting force). To improve machinability, use sharp carbide tools, lower cutting speeds (100–120 m/min vs. 150–180 m/min for P20), and high-quality cutting fluid.
3. Do I need to passivate JIS S136 after machining?
Passivation (treating with nitric acid) is optional but recommended—especially for medical or food molds. It strengthens the chromium oxide layer, enhancing corrosion resistance by 20–30%. For non-critical applications (e.g., consumer product molds), thorough cleaning after machining may be sufficient.
